Method and Apparatus for Manufacturing Titanium Alloys

ABSTRACT

A system and method for producing a metallic ingot using, for example, a VAR furnace, includes a primary crucible receiving a melted metal from a source of metal and collecting the melted metal to for a pool of melted metal, the primary crucible including an overflow lip, a secondary crucible receiving the melted metal from the overflow lip of the primary crucible, the secondary crucible being smaller than and electrically isolated from the primary crucible, and a withdrawal device withdrawing the molten metal, solidified by cooling, from the secondary crucible in the form of solidified ingots, wherein the solidified ingots have a smaller diameter than a diameter of the source of metal. A cutting device periodically cuts the withdrawn solidified ingots as they are withdrawn from the secondary crucible.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of co-pending Provisional Patent Application Ser. No. 61/176,340 entitled “Method and Apparatus for Manufacturing Titanium Alloys”, filed on May 7, 2009, the entire disclosure of which is incorporated by reference herein.

FIELD OF THE INVENTION

The present invention is directed toward the processing and manufacture of metals and, more particularly, toward the processing and manufacture of titanium and titanium alloys.

BACKGROUND OF THE INVENTION

For over sixty (60) years, titanium and titanium alloys have been found to be of great use in industry. Initially, such use was found in aerospace applications, and later in industrial applications such as the chemical process industry, oil and gas industry, medical industry, etc. The unique combination of titanium's strength-to-weight (useful in aerospace applications) and corrosion resistance (useful in chemical process, oil and gas, and medical applications) have created a large demand for its use. As a result, the production of titanium has grown consistently over the years. Due to advances in the aerospace industry and aircraft manufacturing techniques over the past ten to fifteen (10-15) years, the use of titanium products in aerospace applications in the near future is expected to grow much more rapidly than at any period in its history.

Manufacturing methods for making various small parts from titanium and titanium based alloys are well established, from the initial melting of the alloy through the final small part fabrication. Such small parts may include, for example, fasteners, fittings, and small machined, forged or formed parts. Titanium has a very diverse use, which varies from medical devices, such as hip implants, to oil well logging tools. Typically, the titanium products are produced first by normal ingot forging and other hot working operations, and then by cold drawing and machining from titanium mill products, such as billets or bars, or even smaller mill products, such as hot rolled coil. Hot rolled coils of titanium are spooled coils of metal where the diameter of the metal in the coil typically may be from 0.200″ (5.08 mm) up to 0.875″ (22.225 mm) in diameter. This latter category of coil (hot rolled coil) has become very significant over the past few years as new generations of commercial and military aircraft employ more composite materials in their (the aircraft's) fabrication.

Previously used aluminum fasteners are not compatible with the increased use of composite materials due to the possibility of increased corrosion between aluminum and composite (graphite) materials. This has dramatically increased the already significant use of titanium fasteners in the material systems for the new aircrafts. As an example, previous generations of commercial aircraft, such as the Boeing 737, 747 and 767, as well as other commercial aircraft, variably used between 20,000 and 40,000 pounds (9,080 and 18,160 kg) of titanium alloys in their airframes, including the fasteners. The newer generation aircrafts, such as the Boeing 787, will use in excess of 200,000 pounds (90,800 kg) of titanium in their airframes, and this is excluding the engine. Newer generations of commercial aircraft produced by, for example, Airbus Industries, also will see a similarly significant increase in the use of composites and, as a result, will thereby require large quantities of titanium fasteners. Most of the larger use of titanium is comprised of various titanium alloy fastener systems to secure the composite skin of the aircraft to the airframe.

Current methods for making small diameter titanium and titanium alloy (e.g., NiTi alloy) parts, as well as some steel, cobalt or nickel based alloy parts, or even other reactive metal parts (e.g., zirconium, vanadium, titanium aluminum, etc.) are generally as described below

Titanium and titanium alloys are melted using either virgin titanium sponge and other alloying metals and/or master alloys, or by using scrap, or by using some combination of virgin materials and scrap depending upon the demands of the particular application. There are a number of melting practices that can produce a useable titanium or reactive metals ingot that can then be processed into a mill product configuration suitable for the kind of applications generally discussed above. These melting practices currently include, for example, Vacuum Arc Melting, Plasma Arc Furnaces or Electron Beam Furnaces, or Cold Walled Induction Skull Melting Furnaces.

Vacuum Arc Melting of elemental titanium sponge and alloying elements starts with compacted forms of sponge and scrap that are welded together or otherwise attached and then suspended in a vacuum chamber and remelted. A typical configuration of a Vacuum Arc Remelt (“VAR”) furnace can be seen in the upper portion of the attached FIGS. 1-2. The melting occurs when the remelt electrode, which is composed of virgin titanium or other reactive metals or scrap or some combination of virgin metal and scrap, is placed inside a vacuum chamber which also contains a copper crucible as part of the vacuum chamber. The electrode is lowered into the copper crucible until it strikes the bottom of the crucible which is at the opposite electrical potential. This causes an arc between the crucible bottom, or anode, and the remelt electrode, which is the cathode in the electrical circuit, which generates sufficient heat to cause the electrode, which becomes sacrificial, to melt and to drip molten titanium metal onto the bottom of the crucible, thus forming a new ingot which is more homogenous than the remelt electrode from which it derived its raw material. Until this point, the new “VAR” ingot that is crystallized inside the copper crucible has always been larger in diameter than the remelt electrode from which it draws its source metal prior to striking an arc. For example, see U.S. Publication No. US 2006/0230876 and Zanner, “Vacuum Arc Remelting—An Overview”. This is always true because the Vacuum Arc Remelting process requires a space, or annulus, between the wall of the copper crucible and the electrode to be remelted in order to prevent the to be remelted electrode from coming into contact with the copper crucible side wall and injuriously and prematurely arcing there. This may cause the melt to progress improperly or otherwise operate in a non-controllable fashion and prematurely end the melt and possibly damage the copper crucible or even the VAR melt equipment itself. The only proper location for the arc in the Vacuum Arc Remelt furnace system is off the bottom of the remelt electrode, just above the crucible bottom and the pool of the newly melted and crystallized ingot. Thus, the newly melted ingot may be, for instance, thirty inches (30″) (76.2 cm) in diameter, whereas the source remelt electrode from which it was formed may only be, for instance, twenty-four inches (24″) (60.96 cm) or twenty-six inches (26″) (66.04 cm) in diameter.

This Vacuum Arc Remelting process may be performed several times in order to progressively cause the new ingot to become more homogenous and to contain fewer defects, and, in the case of forging and hot working efficiency, to provide a larger single ingot of metal. With each sequential remelt in the Vacuum Arc Remelt furnace, the crystallized ingot that is formed in the copper crucible becomes larger in diameter for the reason mentioned in the previous paragraph.

In addition to Vacuum Arc Remelting, other melt techniques for producing titanium ingots are to use Plasma Arc furnaces or Electron Beam Furnaces, or Cold Walled Induction Skull Melting Furnaces. Generally, although not in all cases, these furnaces are used to consolidate various raw material forms, such as titanium sponge, master alloy or different geometries of scrap, so as to easily produce a remelt electrode that can be subsequently remelted in a VAR furnace. In some cases for the Plasma and Electron Beam Furnaces, certain customer and industry specifications allow for the direct use of as-melted Plasma Arc melted ingots, or Electron Beam melted ingots. This is typically not now the case, however, where the final titanium products are used in critical aerospace applications, such as airframes and fasteners for airframes, and in the medical products industry. The appropriate industry applications (specifications) call for multiple vacuum melting cycles, the last of which must be Vacuum Arc Remelting.

As a result of the above, titanium, reactive metals and other industries require VAR melting as the final of multiple melts in an ingot melting cycle. Progress in the melting of these products has focused around producing larger VAR ingots, thus giving the ingot producer and processor more efficient quantities of metal in one heat (the product of one ingot melted at one time) for the subsequent production of smaller diameter round products to be used in products, such as fasteners or small round parts, for use in, for example, the medical or chemical or oil and gas industries. Progress has also focused on automating VAR melting, introducing the use of computers, melt profiles, start up and melt completion procedures, and related measures to assist with the production of larger ingots that melt both larger amounts of metal at one time, and also more complex alloys.

Complex titanium and other reactive metal alloys exhibit tendencies to not remain completely homogenous during the melt cycle, especially as the ingots melted become larger. Thus, the progress in melt management and scale is partially offset by the production of a not immediately useful titanium or reactive metal ingot due to ingot homogeneity problems caused by large melt pools that have a tendency to see various alloy components segregate upon the slow cooling that takes place in the center of the larger ingot. This problem may be overcome by extended solid state heating and homogenizing of the complex ingot, but it is costly and requires a significant amount of energy, in the form of natural gas or electricity, for the heating and homogenizing of the ingot.

All large titanium and reactive metal ingots experience multiple hot working and deformation cycles to reduce their diameter from the as-melted ingot diameter at, for instance, thirty inches (30″) (76.2 cm) in diameter to the diameter of a hot rolled coil at, for instance, approximately 0.450″ (11.43 mm) in diameter, from which an aerospace fastener might be fashioned. Specifically, an ingot that is thirty inches (30″) (76.2 cm) in diameter that has been final melted in a VAR will undergo processing very similar to that described below

First, the ingot is conditioned by grinding to remove artifacts of the melt on the surface of the ingot. There is very significant capital equipment and yield loss involved here. Yield loss may typically vary from approximately 2% to 5% of the ingot weight.

Second, the ingot is heated to a very high temperature, on the order of approximately 50-70% of the melt point of the alloy, roughly 1,800-2,200° F. (982.22-1,204.44° C.), and forged from an as-melted ingot to an intermediate form. The intermediate form is generally around eighteen inches (18″) (45.72 cm) round, octagonal or square by about four times the length of the original thirty inch (30″) (76.2 cm) ingot. Significant capital equipment in the form of very large forging presses or rolling and cogging mills, and large and multiple heating furnaces are involved for this step in order to have a product that is ready for the coil mill rolling operation.

Third, the intermediate eighteen inch (18″) (45.72 cm) billet from the parent ingot is conditioned by surface grinding to remove defects which could cause problems in the subsequent hot working steps.

Fourth, the eighteen inch (18″) (45.72 cm) billet is heated to a high temperature, or red heat, and forged again to about nine inches (9″) (22.86 cm) round or square, again about a 4:1 reduction of cross-sectional area.

Fifth, the nine inch (9″) (22.86 cm) billet is conditioned by grinding to remove deleterious surface imperfections prior to the next processing step. There is additional yield loss at this step.

The sixth operation involves the heating and forging or rolling of the nine inch (9″) (22.86 cm) billet to a billet approximately four inches (4″) (10.16 cm) round. This again is a 4:1 reduction of the cross-section of the nine inch (9″) (22.86 cm) billet. Again, heating and forging or roll cogging operations here are costly and repetitive.

The seventh operation is the surface conditioning of the approximately four inch (4″) (10.16 cm) round bar or billet in order to have its surface in smooth condition as it is processed into a coil for further subsequent processing into small parts, such as fasteners.

The eighth and generally the final hot working operation is the rolling of the nominally four inch (4″) (10.16 cm) round bar or billet to a coil that can then be processed by methods other than hot working methods (e.g., cold working, drawing, and machining) to produce products such as fasteners and other small round components.

It can clearly be seen that each time the ingot is heated, significant energy is expended. Also, as a result of the various grinding steps and related conditioning of the ingot and intermediate forged billets, a large portion of the starting ingot is lost, or yielded, due to the repetitive oxidizing of the ingot or billet surface at high temperatures and the necessity of removing this layer along with any cracking or defects induced in the multiple of hot working steps. It is generally accepted in the industry that the loss from an as-melted VAR titanium ingot to the point at which a four inch (4″) (10.16 cm) round billet is formed is on the order of 15% of the starting ingot weight. This means that only 85% of the starting ingot is available to commence the final hot working cycle, whereby the four inch (4″) (10.16 cm) bar or billet is converted to a small diameter round or hot rolled coil.

The present invention is directed at overcoming one or more of the above-mentioned problems, and reducing the economic loss from multiple heating, forging and grinding operations.

SUMMARY OF THE INVENTION

In one embodiment, a system for producing a metallic ingot is provided which includes a primary crucible receiving a melted metal from a source of metal and collecting the melted metal to form a pool of melted metal, the primary crucible including an overflow lip, a secondary crucible receiving the melted metal from the overflow lip of the primary crucible, the secondary crucible being smaller than and electrically isolated from the primary crucible, and a withdrawal device withdrawing the melted metal, solidified by cooling, from the secondary crucible in the form of solidified ingots, wherein the solidified ingots have a smaller diameter than a diameter of the source of metal. A cutting device periodically cuts the withdrawn solidified ingots as they are withdrawn from the secondary crucible.

The system may include a heat source provided above the secondary crucible for keeping the melted metal at the top molten. In one form this heat source includes a non-consumable electrode.

The source of metal may include titanium or a titanium alloy or other reactive metal. In one form, the source of metal includes a consumable electrode of the metal which is melted using a VAR furnace.

In another embodiment, a method of manufacturing a metal is provided, which includes the steps of melting a source of metal to form a pool of melted metal in a primary crucible, transferring the melted metal from the primary crucible to a secondary crucible, the secondary crucible being smaller than and electrically isolated from the primary crucible, cooling and solidifying the melted metal, and withdrawing the cooled and solidified metal from the secondary crucible. The withdrawn cooled and solidified metal is then periodically cut as it is withdrawn from the secondary crucible to form solidified ingots.

The transferring step may include directing the melted metal from the primary crucible via an overflow lip formed in the primary crucible. As the primary crucible is filled with the melted metal, the overflow lip directs the flow from the primary crucible to the secondary crucible. Preferably, the secondary crucible is heated to keep the melted metal at the top molten. In one form, this heating is accomplished using a non-consumable electrode.

The source of metal may include titanium or a titanium alloy or other reactive metal. In one form, the source of metal includes a consumable electrode of the metal which is melted using a VAR furnace.

In a further embodiment, a system for producing a metallic ingot is provided including a crucible receiving a melted metal from a source of metal and collecting the melted metal to form a pool of melted metal, wherein the crucible includes a hole formed the bottom thereof, the hole defined by sidewalls extended downward from the crucible bottom, and a withdrawal device withdrawing the melted metal, solidified by cooling, from the crucible hole in the form of solidified ingots, wherein the solidified ingots have a smaller diameter than a diameter of the source of metal. A cutting device is provided which periodically cuts the withdrawn solidified ingots as they are withdrawn from the crucible hole.

In yet a further embodiment, a method of manufacturing a metal is provided including the steps of melting a source of metal to form a pool of melted metal in a crucible, wherein the crucible includes a hole formed the bottom thereof, the hole defined by sidewalls extended downward from the crucible bottom, providing a starter piece of metal in the crucible hole, wherein the starter piece of metal is compositionally the same as the source of metal, allowing the melted metal to pool in the crucible, and upon the pooled melted metal reaching approximately one to four inches (1-4″) (2.54-10.16 cm) in height in the crucible, withdrawing the starter piece of metal from the crucible hole, wherein the starter piece of metal has cooled and solidified metal attached to it. The withdrawn cooled and solidified metal is periodically cut as it is withdrawn from the crucible hole to form solidified ingots.

It is an object of the present invention to continuously produce metal ingots having a diameter suitable for coil, bar or rod rolling while allowing the melt to continue uninterrupted.

It is a further object of the present invention to minimize the number of melts required to produce metal ingots having a diameter suitable for coil, bar or rod rolling while still honoring industry specifications calling for VAR melting as the final melt in a multiple melt cycle.

Other objects, aspects and advantages of the present invention can be obtained from a study of the specification, the drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a first embodiment of the present invention incorporated into a VAR furnace; and

FIG. 2 illustrates a second embodiment of the present invention incorporated into a VAR furnace.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to methods and apparatus for producing smaller cross-sections (diameters) of as-melted ingots from a larger input remelt electrode by, for example, VAR melting an ingot that can subsequently be processed directly, or with a maximum of one additional hot working cycle, into smaller mill products of titanium and other metal products. These smaller cross-sectional or smaller round ingots will be continuously withdrawn from a crystallization crucible or mold and continuously removed in order to allow the melt, which is being fed by a larger cross-section remelt electrode or ingot, to continue to progress uninterrupted. These smaller ingots are melted from larger remelt electrodes in, for example, a conventional VAR furnace and progressively withdrawn from the copper crucible, or a companion copper crucible, as the ingot crystallizes. This allows for the continued use of larger lot sizes (melt sizes) while producing a final melt ingot that is closer to the input diameter for coil rolling or bar or rod rolling mills. This also completely honors all industry standards and specifications calling for VAR melting as the “final” melt in a multiple melt cycle to produce a final ingot. The inventive process produces a very large energy savings, capital equipment savings, and yield savings of the titanium (or other alloy ingots) processed in this manner.

The present invention allows for the production by VAR melting and continuous withdrawal of an ingot smaller in diameter than the input remelt electrode/ingot from which it was melted.

The present method and apparatus can produce, for example, a four inch (4″) (10.16 cm) round (or similar diameter or size bar or billet) that is ready to be coiled directly. The present method and apparatus utilizes less than 30% of the energy that conventional VAR ingots normally utilize in order to be processed from a larger ingot into a shape that is ready for coiling. The present invention also provides for these smaller diameter ingots to be produced by continuously withdrawing the ingot from the crystallization chamber by unique means as described below.

In the patent literature, the term “continuous casting” was used to mean that the process of melting itself progressed continuously from the beginning to the end. This is self evident.

In the present inventive method and apparatus “continuous casting” as discussed herein, means the continuous melting, casting, and withdrawing of an ingot from its crystallization mold as the melt progresses. This continual production, withdrawal and removal of an ingot in the context of VAR melting are completely new and previously unpracticed. It does not mean or is meant to mean the continuous casting and filling of a static melt crystallization crucible (or other cooling chamber or receptacle) as is now the established practice in VAR melting. In U.S. Pat. No. 5,103,458, the terms “withdrawal of the ingot” and “withdrawing the ingot from the mold” are used. To be clear, this is a reference to the standard technique of completing a VAR melt cycle and “withdrawing” or removing the ingot from the melt apparatus once the melt process is complete and an ingot has been formed in the melt crucible. Conversely, the present invention provides for the continuous withdrawal and removal of the as-melted smaller diameter ingot while the melt process is continuing.

The present invention also provides for an as-melted ingot to be produced that may be then introduced, with a minimum of handling or conditioning, and with potentially no additional hot working cycles, to a continuous coil or bar rolling mill that will reduce the input approximately four inches (4″) (10.16 cm) in diameter to a small cross-section hot rolled coil or multiple straight strands of product.

The present invention also provides for a removal method from the bottom of the copper crucible or from the side of the copper crucible that may be controlled via a logic loop between a computer and various controllable aspects and parameters of the ingot melt furnace.

The present invention also provides for parts of the removal apparatus that are new and unique to VAR melting and that enable the small diameter ingot to be easily and continuously removed from the molten melt pool without undue difficulty, and which produces a high quality as-melted ingot with minimal surface perturbations and defect injuries to the subsequent coil or small round rolling cycle.

The present invention and description of continuous ingot removal methods from the liquid melt may be processed in the substantially vertical plane or the substantially horizontal plane. These examples discussed herein are in the substantially vertical plane.

The present invention provides for the intermittent cutting and or removal of the as-cast vertical or horizontal continuously melted and withdrawn ingot under vacuum, and while the melt continues.

The present method and apparatus are new and unique in that prior to the present invention, a VAR melted ingot has never been removed continuously from the melt crystallizer while the electrode to be remelted is arcing, dripping molten metal into a crystallizer mold and thereby remelted under vacuum.

As can be seen in the upper portion of the attached FIG. 1, the current state of the art for VAR melting provides for the production of an as-melted ingot larger than the diameter of the sacrificial remelt electrode/ingot. Those knowledgeable in the art will note that virtually as soon as molten titanium comes into contact with the copper walls of a crystallization chamber, whether it is a crucible in VAR melting or a withdrawal mold in Plasma and/or Electron Beam melting, the surface of the molten titanium in contact with the copper crucible immediately forms a meniscus or thin layer of solidified titanium metal. The progressive growth of this meniscus and additional layers of solidified titanium next to the meniscus at the contact zone of the molten metal with the crucible wall presents a large impediment to the facile withdrawal of the ingot from the crystallization container, whether it is a VAR crucible or a withdrawal mold as practiced in Electron Beam, Plasma or other forms of reactive metals melting. Solidification conditions in this case (in the area of the meniscus) are quite sensitive to the details in the contact zone (see Zanner, “Vacuum Arc Remelting—An Overview”). The contact zone between the ingot and copper crystallization mold or crucible then becomes the single most important focus for a successful withdrawal technique. Failure to properly address the issue of meniscus formation and solidification of the reactive (or any metal or alloy) metal will cause the ingot to become lodged in place or otherwise immoveable. Alternatively, if the smaller diameter ingot being withdrawn is moveable, the progressive withdrawal of a useable ingot may be inhibited by unacceptable tearing of the ingot surface as the withdrawal puller exerts axial tensile forces on the smaller diameter ingot to remove it from the melt pool.

There are then two primary issues to be overcome in order to successfully withdraw a smaller diameter ingot from a larger diameter input VAR remelt electrode. The first is to achieve a steady state of withdrawal that overcomes the issues noted above regarding the formation of a meniscus and dealing with the issues that arise as a result of the meniscus.

The second is to maintain sufficient heat on the surface of the smaller ingot being withdrawn so that the edges and top of the smaller ingot closest to the side walls of the withdrawal crucible do not prematurely solidify and allow for additional hot metal to flow over this solidified area, thus forming what are known as “shuts” or “cold shuts”. These cold shuts need to be removed after melting in order to have a useable ingot diameter that will not crack, split or otherwise fail during subsequent hot rolling to rod, coil or bar operations. The needed removal of these cold shuts, however, reduces the useable diameter of the ingot significantly. For instance, if a cold shut were to migrate 0.25″ (6.35 mm) in from the edge of a four inch (4″) (10.16 cm) round ingot (i.e., off the radius), then 0.50″ (12.7 mm) of the four inch (4″) (10.16 cm) diameter ingot would require removal from the diameter in order to have a smooth defect free bar or billet that is ready for the hot rod, bar or coiling operation. This would result in a yield loss of the three and one-half inch (3.5″) (8.89 cm) bar divided by the diameter of the four inch (4″) (10.16 cm) diameter as-melted ingot, or 9.616 in²/12.56 in² (62.504 cm²/81.64 cm²), or a 23.44% yield loss at this stage of the melting alone. From an economic standpoint, this is unacceptable. For this reason, if the smaller diameter ingot is removed as discussed in the method below, the surface of the ingot in the withdrawal crucible needs to receive additional heating sufficient to minimize the edge effects of cold shuts.

Additional heating to the top of the smaller ingot being withdrawn may come from a number of sources. They potentially are:

-   -   A plasma torch that could heat the top of the smaller diameter         ingot in the withdrawal mold.     -   An Electron Beam Gun that could heat the top of the smaller         diameter ingot being withdrawn from the mold.     -   The cold wall Induction Crucible could be used as the withdrawal         crucible. This cold wall Induction Crucible would be heated by         an induction coil on the outside so as to cause the smaller         diameter ingot inside the crucible to remain hot and/or liquid         until a point where all edge effects or cold shuts will not         possibly form and affect ingot quality.     -   A non-consumable copper electrode that could provide electrical         heating via an arc directly applied onto the top of the smaller         diameter ingot being withdrawn, delaying its solidification near         the top and allowing for a more uniform defect free ingot as it         is withdrawn.

Prior to acceptance or rejection of any of the above techniques, it is important to understand the basic nature of VAR melting. VAR melting takes place, in practical operation, when the remelt electrode, or the ingot to be remelted, is lowered to short itself (note that the bottom of the copper crucible is the anode in the circuit and the remelt electrode is the cathode in the circuit from which the melting arc is created) either against the bottom of the copper crucible or upon metal that is of a similar composition to the metal being remelted, be it a “strike plate” of similar composition, or a small pile or agglomeration of particles of a like composition metal that has been placed on the bottom of the copper crucible. Once the aforementioned short takes place, the remelt electrode is immediately withdrawn a short distance to create a gap across which the short is replaced by a melting arc that is generated as long as the gap is not too large. Typically, the gap will be on the order of 6-12 mm (0.24-0.48″). Ideally, the melting arc generates a uniform plasma by Joule heating (see Zanner, “Vacuum Arc Remelting—An Overview”). This arc and/or plasma is highly susceptible to stray or unintended magnetic fields that may disrupt, disturb or otherwise create large non-uniformities of the arc, the metal pool formed under it, and/or the uniform process of the melt. So care must be exercised when evaluating a heating technique for the top of the small ingot being withdrawn so that the technique does not generate strong magnetic fields or currents that would interfere with the uniformity of the primary arc melting taking place under the remelt electrode and between that electrode and the crucible bottom or molten pool that has formed on the crucible bottom.

Since meniscus formation and associated crystallization are completely unavoidable in the copper crucible ingot formation system, the success of the withdrawal technique becomes dependent upon incorporating and minimizing those effects, and building a model and process control capability for ingot withdrawal that anticipates them. The model should anticipate:

-   -   Meniscus tearing from the axial tensile ingot removal forces.     -   Minimizing the amount of the meniscus tearing on each         progressive axial pull.     -   Heating of the top of the small ingot to be withdrawn         continuously from the melt in a manner sufficient to maintain         the meniscus sufficiently thin to be manipulated as necessary         for the formation and withdrawal of the small ingot.

This model must also anticipate that the ingot must be withdrawn incrementally, regularly and progressively in such a manner that only minimally overcomes the strength of the meniscus and associated crystallized metal build-up in the copper crucible contact area. The withdrawing of the ingot should disrupt the meniscus only enough for the ingot to be pulled down a small increment, allowing for the formation of a new meniscus immediately as molten metal from the pool flows to meet the cold copper crucible wall where the tear occurred, as a result of the tearing and downward removal of the ingot. The withdrawing needs to be done in small increments and repeated with a frequency sufficient to satisfy the model criteria.

With the foregoing discussion of VAR melting, contact zone issues, meniscus formation and metal crystallization, and small ingot heating requirements provided as infrastructure for the presentation of the embodiments of the present invention, two specific embodiments are proposed as definitive methods for implementing the present invention. However, these embodiments are for illustrative purposes only and are not meant to be limiting. One skilled in the art will appreciate that other embodiments can be implemented that fall within the spirit and scope of the present invention.

The first embodiment is shown in FIG. 1, in which the upper portion depicts a conventional VAR furnace having an electrode feed drive 1, a furnace chamber 2, a melting power supply 3, bussbars/cables 4, an electrode ram 5, a water jacket 6, a vacuum suction port 7, an X-Y axis adjustment 8, and a load cell system 9. The VAR furnace operates in conventional format to melt the remelt electrode or ingot. The first embodiment contemplates the withdrawal of an ingot smaller than the to be remelted feed ingot or electrode by overflowing the VAR copper crucible, shown at 10 in FIG. 1. The copper crucible 10 is preferably a shallow water cooled crucible. Metal melts off of the electrode 12 and drips into the shallow crucible 10. The copper crucible 10 includes a spout 14 that directs the molten metal from the primary crucible 10 to a secondary, smaller copper crucible 16. Heat from the arc keeps the metal molten until it fills the crucible 10 and spills out the spout 14 and is collected into the secondary crucible 16. Preferably, the secondary crucible 16 is a collar chill crucible and has a withdrawal mechanism in the bottom of it. The smaller, secondary crucible 16 needs to have the following characteristics.

The secondary crucible 16 should be close enough to the primary VAR copper crucible 10 so that metal flows or falls easily into it. The secondary crucible 16 should also be electrically isolated from the primary copper crucible 10. Previous methods of withdrawal ingot technology for both Electron Beam and Plasma Melting do not provide for electrical isolation of any smaller crucible from which any smaller ingot is withdrawn. In these previous cases, the withdrawal crucible and feed crucible or hearth have been made of either a single integral manufacture or, if not, have been connected with metal connectors in such a fashion as to provide a single electrical circuit for the complete assembly. In the case of VAR melting, the consideration of electrical isolation of a smaller crucible from a larger feed crucible had not been addressed in any previous literature or embodied in any previous design. Electrical isolation is critical in VAR melting due to the requirement for maintaining a completely different arc heating power level for the smaller withdrawal crucible than the primary melt crucible. Failure to provide a separate circuit would allow the smaller crucible to become part of the larger melt crucible electrical ground, and thus the arc path of the larger VAR remelt crucible circuit, thereby interfering with the lower power arc required for the smaller withdrawal crucible.

As an example, the primary Vacuum Arc Remelt electrode may be between sixteen inches (16″) (40.64 cm) and twenty-six inches (26″) (66.04 cm) in diameter, and would then experience a melt current, I arc, on the order of 14KA to 26KA with a voltage, V, of approximately 26-40 volts. The secondary, smaller withdrawal crucible would require a current, I arc, for heating the top of the ingot of between 4KA and 8 KA, and a voltage, V, of between 24 and 30 volts. It is critical that these different electrical demands and requirements remain independently controllable and isolated. Hence the reason for electrical isolation of the primary VAR crucible 10, and the smaller, withdrawal crucible 16.

The collection and withdrawal crucible 16 for the smaller ingot requires an additional source of heating energy to prevent the undue formation of cold shuts (as discussed above) on the new smaller ingot as it is being formed and withdrawn. An evaluation of all of the energy sources available (including Plasma Arc, Electron Beam, Cold Wall Induction Heating, and Vacuum Arc Heating) determined that an additional Vacuum Arc source of heat, on a different electrical circuit so as to be independently controlled from the primary Vacuum Arc in the larger remelt crucible, would solve the problem of secondary heating and also meet the following three primary criteria for secondary heating of a withdrawal crucible:

-   -   1) That the secondary heating arc generate minimal, little or no         magnetic field interference with the nearby primary arc used for         the melt down of the larger remelt electrode in the primary         Vacuum Arc Remelt crucible.     -   2) That the secondary heating arc provide sufficient heating to         the surface of the smaller, to be withdrawn ingot so as to         eliminate premature cooling and freezing of the surface of the         smaller ingot, thus eliminating or reducing cold shuts or other         deleterious imperfections on the surface of the withdrawn ingot         to an acceptable level.     -   3) That the secondary heating arc meet the letter and spirit of         all relevant specifications that demand Vacuum Arc Remelting as         the final melt in a series of melts or melt cycles.

These requirements can be met, for example, with a small, non consumable water cooled copper electrode 18 that arcs in a fashion essentially similar to the arc from the primary melt down arc, but is not consumed and operates at a lower power only sufficient to heat the top of the smaller electrode as discussed. However, one skilled in the art will appreciate that other heat sources may be used above the secondary crucible 16 without departing from the spirit and scope of the present invention.

Then, with the provisions made for a first embodiment of the present invention, the following elements are present:

-   -   A primary copper crucible 10 which operates in a conventional         VAR fashion by collecting the remelted metal that drips off of         the remelt electrode 12, thereby forming a pool of melted metal.     -   An overflow lip 14 in the primary copper crucible 10 that allows         metal from the pool to drip or flow out of the crucible 10 and         into a smaller collection crucible 16 that is part of a         completely separate electrical circuit (apart from the main         crucible).     -   A smaller collection crucible 16 that collects the molten metal         flowing from the VAR main crucible 10 and forms it into an ingot         of smaller diameter that is subsequently withdrawn to form a         smaller diameter ingot.     -   A non-consumable water cooler copper electrode 18 that arcs to         the top of the small ingot being withdrawn so as to keep the top         molten and free of shuts or problems that may reduce its         economical use.

A programmable logic controller (“PLC”) automatic control loop may be provided that causes the withdrawal rate of the smaller ingot, the melt rate and power levels of the larger remelt ingot and the smaller withdrawal ingot, and the power levels of the auxiliary heating arc on the smaller remelt ingot, all to be integrated. This is done to ensure that the rate at which the smaller withdrawal ingot is being removed from the smaller copper crystallization crucible/mold 16 is equal to the rate at which the molten metal is dripping off the larger remelt electrode 12, into the pool of the large copper crucible 10, over the lip 14 and into the smaller withdrawal crucible 16. This ensure continuous operation and withdrawal of the smaller diameter ingot.

A withdrawal device, show at 20, is provided at the bottom of the smaller crucible 16 and withdraws and removes the ingot from the small crystallization crucible or mold 16 continually during the conduct of the melt in the direction of the arrow. The withdrawal device 20 withdraws the ingot out of the smaller crucible 16 in a regular progressive fashion. A cutting device 22 is provided which periodically cuts the ingot and moves it aside so that the entire process can continue uninterrupted.

A second embodiment of the present invention is shown in FIG. 2 with like elements from FIG. 1 indicated with the same reference number and those elements requiring modification indicated with a prime. The second embodiment approaches the withdrawal of a smaller ingot from a larger VAR melt crucible from a more straightforward method. The upper portion of FIG. 2 again depicts a conventional VAR furnace having an electrode feed drive 1, a furnace chamber 2, a melting power supply 3, bussbars/cables 4, an electrode ram 5, a water jacket 6, a vacuum suction port 7, an X-Y axis adjustment 8, and a load cell system 9. The VAR furnace operates in conventional format to melt the remelt electrode or ingot. In the second embodiment, the withdrawal device 20′ is situated in the bottom of the primary VAR remelt crucible 10′. A secondary, smaller, electrically isolated crucible as described in the first embodiment is not required. In the second embodiment, the withdrawal device 20′ for the smaller ingot is built into the primary VAR crucible 10′. The VAR crucible 10′ then acts as its own tundish, filling the withdrawal ingot from the molten metal directly above it and directly underneath the melt pool formed by the dripping metal off the remelt electrode 12.

As an example, if the bottom of the VAR crucible is round and is approximately twenty-four inches (24″) (60.96 cm) in diameter, a four inch (4″) (10.16 cm) hole in the bottom of the crucible 10′ could be fabricated. The four inch (4″) (10.16 cm) hole would extend downward, out of the bottom of the crucible 10′ and for some distance, as shown at 24. The sides of the withdrawal hole 24 are preferably copper, as would be expected from a standard withdrawal crucible.

At the commencement of a VAR melt cycle, a starter piece of titanium or metal that is compositionally the same as the to be remelted ingot would be inserted into the four inch (4″) (10.16 cm) hole and fixed to a pulling device 20′ that is capable of withdrawing the smaller ingot out of the bottom of the VAR crucible 10′ in the direction of the arrow as it is filled or built up from the melt pool directly overhead. The VAR melt is then initiated, and molten drops of metal will commence accumulating against the bottom of the crucible 10′ and forming the previously discussed meniscus. The molten drops of the metal would also fall onto the starter bar in the ingot puller and initially solidify. However, as is the case with titanium, the heat transfer of titanium is so low that there would, with the continuation of the arc current from immediately above the puller, soon be some melting on the face of the puller bar. The melt would then proceed until approximately one to four inches (1-4″) (2.54-10.16 cm) of molten metal fills the bottom of the VAR crucible 10′. At this point, the pulling of the small diameter ingot would then commence, thus withdrawing the ingot from the bottom of the crucible 10′. As previously described with respect to the first embodiment, a cutting device 22 is provided which periodically cuts the ingot and moves it aside to that the entire process can continue uninterrupted.

While the present invention has been described herein with particular reference to the drawings, it should be understood that various modifications could be made without departing from the spirit and scope of the present invention. Those skilled in the art will appreciate that various other modifications and alterations could be developed in light of the overall teachings of the disclosure. The presently preferred embodiments described herein are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims and any and all equivalents thereof. 

1. A system for producing a metallic ingot comprising: a primary crucible receiving a melted metal from a source of metal and collecting the melted metal to for a pool of melted metal, the primary crucible including an overflow lip; a secondary crucible receiving the melted metal from the overflow lip of the primary crucible, the secondary crucible having heated side walls, being smaller than and electrically isolated from the primary crucible and having an opening through which the molten metal may pass as the metal solidifies into an ingot, the opening having a smaller diameter than a diameter of the source of metal; and a withdrawal device withdrawing the melted metal, solidified by cooling, from the secondary crucible in the form of a solidified ingot
 2. The system of claim 1, further comprising a cutting device periodically cutting the withdrawn solidified ingot as the ingot is withdrawn from the secondary crucible.
 3. The system of claim 1, further comprising a heat source provided above the secondary crucible for keeping the melted metal at the top molten.
 4. The system of claim 3, wherein the heat source comprises a non-consumable electrode.
 5. The system of claim 1, wherein the source of metal comprises titanium or a titanium alloy.
 6. The system of claim 1, wherein the source of metal comprises a consumable electrode of the metal.
 7. The system of claim 1, wherein the source of metal is melted using a VAR furnace.
 8. A method of manufacturing a metal comprising: melting a source of metal to form a pool of melted metal in a primary crucible; transferring the melted metal from the primary crucible to a secondary crucible, the secondary crucible having sidewalls and being smaller than and electrically isolated from the primary crucible; heating the sidewalls of the secondary crucible; allowing a portion of the melted metal to cool and solidify within the secondary crucible; and continuously withdrawing the cooled and solidified metal from the secondary crucible.
 9. The method of claim 8, further comprising periodically cutting the withdrawn cooled and solidified metal as it is withdrawn from the secondary crucible to form solidified ingots.
 10. The method of claim 8, wherein the transferring step comprises directing the melted metal from the primary crucible via an overflow lip which allows the melted metal to flow from the primary crucible to the secondary crucible.
 11. The method of claim 8, further comprising heating the secondary crucible to keep the melted metal at the top molten.
 12. The method of claim 8, wherein the source of metal comprises titanium or a titanium alloy.
 13. The method of claim 8, wherein the source of metal comprises a consumable electrode of the metal.
 14. The method of claim 8, wherein the source of metal is melted using a VAR furnace. 15.-19. (canceled)
 20. A method of manufacturing a metal comprising: melting a source of metal to form a pool of melted metal in a crucible, wherein the crucible includes a hole formed at a bottom thereof, the hole defined by sidewalls extended downward from the crucible bottom; providing a starter piece of metal in the crucible hole, wherein the starter piece of metal is compositionally the same as the source of metal; allowing the melted metal to pool in the crucible; and upon the pooled melted metal reaching approximately 1-2″ in height in the crucible, withdrawing the starter piece of metal from the crucible hole, wherein the starter piece of metal has cooled and solidified metal attached to it.
 21. The method of claim 20, further comprising periodically cutting the withdrawn cooled and solidified metal as it is withdrawn from the crucible hole to form solidified ingots.
 22. The method of claim 20, wherein the source of metal comprises titanium or a titanium alloy.
 23. The method of claim 20, wherein the source of metal comprises a consumable electrode of the metal.
 24. The method of claim 20, wherein the source of metal is melted using a VAR furnace. 